Shortly before or during Tremadoc time the majority of pre-existing trilobite families died and were replaced by new, short-lived groups and early members of dominant Ordovician families. Endemic forms define six faunas separated into northern and southern provinces, without regard for enclosing lithic type. Constancy of distribution from Arenig to Llandeilo implies physical stability. Reduced endemism in the Caradoc and Ashgill probably reflects reduced barriers and broad oceanic circulation.

H. B. Whittington (1993): Morphology, anatomy and habits of the Silurian homalonotid trilobite Trimerus (in Palaeontological studies in honour of Ken Campbell; proceedings of a meeting of the Association of Australasian Palaeontologists, a specialist group of the Geological Society of Australia, Jell,). - Memoir of the Association of Australasian Palaeontologists, 15: 69-83.

Plethopeltis armatus, and species of Leiocorpyphe, are shown to have in common a median ventral cephalic suture and thoracic axial rings with a poorly-defined articulating furrow and a strong transverse ventral ridge. In other characters they differ greatly, and to group them in the Plethopeltidae is open to question; this family may be polyphyletic. The median ventral suture is regarded as a cardinal character of Asaphida, but it is uncertain whether this character had a single origin or arose more than once. Leiocoryphe gemma may be a paedomorphic species derived from some Upper Cambrian trilobite with an effaced cephalon.
H. B. Whittington and C. P. Hughes. (1972): Ordovician geography and faunal provinces deduced from trilobite distribution. - Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences, 263 (850): 235-278.

Abstract: The geography of the Cambrian world depends largely on analysis of distribution patterns of marine faunas dominated by trilobites. Early Cambrian contrasts between east-Gondwanan faunas dominated by redlichiids and Laurentian faunas dominated by olenellids are well known and suggest a significant distance between these continents. On the other hand, slope-and/or cool-water facies contain numerous shared taxa (particularly agnostoids and oryctocephalids in the middle Cambrian). However, these taxa tell us little about geographic relations between the two continents. Apparent similarities among some less common polymerid trilobites, beginning in late early Cambrian time, may contribute significantly to a refined understanding of the geographic relations of these parts of the Cambrian world. However, taxonomic complications have hindered the usefulness of these taxa. A future project for the Cambrian Subcommission might be sponsorship of small international workshops among taxonomists, much like those developed by conodont workers. Such workshops, perhaps focused on stage-size intervals, might be able to reach consensus on a simplified and reproducible polymerid taxonomy of value for biogeographic analysis.

Abstract: Understanding of the calcified composite eyes of trilobites, the oldest preserved visual system, has advanced greatly in recent decades. Three types of trilobite eye occur, the more derived abathochroal and schizochroal types having evolved neotenically from holochroal eyes. Comparative morphology and phylogenetic considerations suggest that all three eye-types were underlain by common developmental systems. So far, understanding of these systems has been based entirely on morphological data from fossils, particularly the way the visual surface grew and the patterning of lens emplacement. Lenses characteristically form a hexagonal array comprising horizontal rows and, conspicuously in schizochroal eyes, dorso-ventral files. Because individual trilobites sometimes have eyes with different numbers of files, file number must reflect the operation of a developmental programme rather than being under immediate genetic control. An empirical developmental model has been devised to describe trilobite eye development, with separate rules dealing with the initiation of lens emplacement, growth and differentiation of the visual surface, and the termination of lens emplacement. Rarely, trilobites may have visual surfaces of normal size, but which lack lenses. This confirms that visual surface growth must have been regulated separately from lens emplacement, and is a feature that cannot be accounted for by the existing developmental model. Such a developmental separation is one of a number of similarities shared with Drosophila, the modem arthropod in which eye development is best understood. Many aspects of eye development are conserved in the Euarthropoda, and in bilaterian metazoans in general. A revised model for trilobite eye development is proposed using extant phylogenetic bracketing, interpreting morphological data from the fossils in the context of the hierarchy of developmental controls now becoming known from living animals. This new model suggests that overall eye shape and size did not require differential growth of the generative zone, as previously thought, and that no separate instruction was needed to specify the termination of lens emplacement. Instead, these features were regulated directly, by controlling the proliferation of cells making up the nascent visual surface. A process documented from Drosophila, which involves the selective inhibition of cells in front of a wave-like front of differentiation, and that is regulated by widely conserved genes, can be used to explain how the trilobite visual surface became differentiated. The model implies also that changes in hormonally regulated developmental pathways known from recent arthropods may have been responsible for the development of abathochroal and schizoehroal eyes, and for heterochronic secondary eye reduction and blindness in trilobites.

Abstract: Exceptionally rich and well-preserved silicified exuviae comprising numerous immature growth stages allow the tracing of detailed morphological and morphogenetical features of the reduced-eyed phacopid species Cryphops? ensae (Richter and Richter, 1926) from the latest Famennian of Thuringia (Central Germany). Ontogenetic features of this species are more similar to those of Phacops than to Trimerocephalus.

Abstract
A pygidium of Brachymetopus ornatus Woodward shows an abnormally developed left side affecting the whole of the pleural area and its rib development. Such an extensive abnormality, which does not extend beyond the axial furrow and shows none of the signs of resulting from injury, is currently unique. It is interpreted as a teratological condition in which a genetic or developmental malfunction resulted in the partial mimicking of thoracic pleurae rather than the construction of the normal pygidial pleural ribs. The effects of this are progressively greater from the posterior to the anterior parts of the affected pleural region.

Two new genera, Poliellaites and Piochella, and six new species, Eokochaspis? cabinensis, Nyella? plana, Poliellaites gloriosa, Piochella peasleensis, Poliella ovala, and Stephenaspis highlandensis, are described from the Susan Duster Limestone and Log Cabin members, Pioche Shale. Kochaspis Resser, 1935, and Kochaspis liliana (Walcott, 1886) are redescribed based on new material. In addition, several species from the Pioche Shale, that are not addressed in our previous reports are discussed in relationship to existing species and local occurrence. The trilobite diversity of the Poliella denticulata Biozone (new) within the study area is 17 genera and 35 species, occurring in five faunal assemblages.

Clay-rich units, locally termed "butter shales," contain the best-preserved trilobites in the richly fossiliferous Cincinnatian Series and likely provide the highest temporal resolution available within these rocks. Sedimentological and taphonomic evidence indicates that the 0.46-m-thick Mt. Orab "butter shale" bed of the Arnheim Formation is composed of a series of stacked event beds, each representing rapid deposition from a flow bearing fine-grained sediment, most likely associated with distal storm processes below storm-wave base. It contains sedimentary structures similar to those of distal mud turbidites, and comprises a total of at least seven, and possibly many more, alternating silt and clay couplets. These clay and silt layers are interpreted to represent the products of different energetic regimes in a series of discrete depositional events accumulated within a common depositional regime. Trilobites within individual clay beds represent census assemblages of animals alive at the same time, and evidence from sedimentology, taphonomy, and stratigraphic architecture are consistent with accumulation of the whole bed within a period from 101 to 103 years. Silt layers of the Mt. Orab events beds are interpreted to represent parautochthonous assemblages, while clay layers, although displaying reorientation of specimens, are interpreted as autochthonous assemblages. Both layers are deposited in a shallower-water environment than the comparable "granulosa" trilobite cluster of the Kope Formation, which represents an autochthonous assemblage with in-situ burial of trilobites.

Asaphus trilobites preserved in tunnel systems of the trace fossil Thalassinoides from the mid-Ordovician (ca. 465 Ma) Holen Limestone, Sweden, are interpreted as the trace makers, enabled by shallow carbonate firm grounds to construct open tunnel networks and develop habitual infaunal behavior. Their in situ preservation confirms an infaunal ethology inferred for some trilobite taxa from functional morphology. We suggest that predation pressure from large omnivorous nautiloid cephalopods ("Orthoceras" Limestone facies) may have triggered ecologic opportunism. In trilobites well adapted for predatory-scavenging behavior as well as excavation, the tunnel networks functioned primarily for protection, possibly assisting in feeding, breathing, and breeding strategies. Previously, "trilobite burrows" have referred to seafloor traces of locomotion, feeding, and resting (Cruziana, Rusophycus). Infaunal, tunneling trilobites provide new evidence of mid-Ordovician partitioning of the skeletal benthos, adding to an ecologic and trophic tier hitherto interpreted as occupied by soft-bodied organisms. Such trilobites also provide an identity for Thalassinoides tracemakers prior to Devonian evolution of decapod crustaceans.